The field of the invention relates to mode transitions in a direct injection spark ignited engine.
In direct injection spark ignition engines, there are two modes of operation that are typically used. The first mode is termed stratified mode where fuel is injected during the compression stroke of the engine. In the stratified mode of operation, the air/fuel ratio is operated lean of stoichiometry. In the second mode of operation, termed homogeneous operation, fuel is injected during the intake stroke of the engine.
During homogeneous operation, the air/fuel can operate either lean or rich of stoichiometry. However, in some circumstances, the operable stratified operation range of lean air/fuel ratios does not coincide with any operable homogeneous, lean air/fuel ratio. Therefore, when switching between these two modes of operation, air/fuel ratio from one cylinder event to the next cylinder event changes in a discontinuous way. Because of this discontinuous change in air/fuel ratio, engine torque is uncompensated, and has an abrupt change.
One method for eliminating abrupt changes in engine cylinder air/fuel ratio is to adjust ignition timing so that abrupt changes in engine torque will be avoided. Another solution is to adjust throttle position to reduce or increase fresh charge flow entering the intake manifold and therefore compensate for changes in engine torque during discontinuous cylinder air/fuel ratio changes.
The inventors herein have recognized disadvantages with the above approaches. Regarding ignition timing adjustments to avoid abrupt changes in engine torque, this method is only applicable when the magnitude of the torque change is small. In other words, the range of authority of ignition timing is limited by engine misfire and emission constraints. Therefore, the approach is not generally applicable.
Regarding throttle position adjustments to prevent abrupt changes in engine torque, controlling flow entering the manifold cannot rapidly control cylinder charge due to manifold volume. In other words, air entering the cylinder is governed by manifold dynamics and therefore there is a torque disturbance when using the throttle to compensate for discontinuous cylinder air/fuel ratio changes. For example, if the throttle is instantly closed and no air enters the manifold through the throttle, cylinder air charge, does not instantly decrease to zero. The engine must pump down the air stored in the manifold, which takes a certain number of revolutions. Therefore, the cylinder air charge gradually decreases toward zero. Such a situation is always present when trying to change cylinder charge using a control device such as a throttle.
An object of the present invention is to allow air/fuel mode transitions in direct injection engines between respective air/fuel regions which do not overlap while preventing abrupt changes in engine torque.
The above object is achieved and disadvantages of prior approaches overcome by a method for controlling an engine during a cylinder air/fuel ratio change from a first cylinder air/fuel ratio to a second cylinder air/fuel ratio, the engine having an intake manifold and an outlet control device for controlling flow from the intake manifold into the cylinder. The method comprises the steps of indicating the cylinder air/fuel ratio change, and in response to said indication, changing the outlet control device.
By using an outlet control device that controls flow exiting the manifold (entering the cylinder), it is possible to rapidly change cylinder charge despite response delays of airflow inducted through the intake manifold. In other words, a rapid change in cylinder charge can be achieved, thereby allowing a rapid change in cylinder air/fuel ratio while preventing disturbances in engine torque.
An advantage of the above aspect of the invention is that unwanted torque changes can be eliminated when abruptly changing cylinder air/fuel ratio.
In another aspect of the present invention, the above object is achieved and disadvantages of prior approaches overcome by a method for controlling an engine during a cylinder air/fuel ratio change from a first cylinder air/fuel ratio to a second cylinder air/fuel ratio, the engine having an intake manifold, an inlet control device for controlling flow entering the manifold, and an outlet control device for controlling flow exiting the intake manifold. The method comprises the steps of indicating the cylinder air/fuel ratio change, and in response to said indication, changing the outlet control device and the inlet control device.
By changing both the inlet and outlet control devices, it is possible to rapidly change the cylinder air charge despite response delays of airflow inducted through the intake manifold. Since the cylinder air charge can be rapidly changed, the cylinder air/fuel ratio change can be compensated and abrupt changes in engine torque can be avoided. In other words, the present invention controls manifold inlet and outlet flows in a coordinated way to allow a rapid change in cylinder air charge regardless of manifold volume. This rapid cylinder air charge change allows the air/fuel ratio to rapidly change while preventing abrupt changes in engine torque, even during abrupt changes in cylinder air/fuel ratio.
An advantage of the above aspect of the invention is that unwanted torque changes can be eliminated when abruptly changing cylinder air/fuel ratio.
Another advantage of the above aspect of the invention is that by using both an outlet and an inlet control device, a more controlled rapid change in cylinder charge is possible.